Direct engraving of a flexography plate 104 (a non-imaged plate shown in FIG. 1) requires carving three-dimensional (3-D), on plate material, directly with a laser system. This is remarkably different from two-dimensional (2-D) imaging techniques that require post processing steps to produce the 3-D features. FIG. 2 and FIG. 3 show a cross section of an imaged flexographic plate 104. Areas 204 represent ink transfer areas, those areas are engraved in a relative shallow depth. Also printable engraved data 304 is shown in FIG. 3. Non-printable areas or non-ink transfer areas 208, are engraved in a significant greater depths than areas 204 and 304, in order to prevent ink transfer from those areas on a printing substrate during the printing process.
This difference introduces several challenges for the laser imaging system:                1. The laser system must have sufficient power to ablate the material; and        2. The laser spot should be small enough to achieve the fine detail required in quality printing.Although high power density does not necessary conflict with laser focusability, from a practical perspective, these lasers offer significantly higher cost per watt of output optical power than broad spot lasers. As a result, it is desirable to operate with broad laser sources, that may produce high output optical power, rather than with small spot sources, that may have high power density but relatively low total power output.        
It is therefore appealing to use a laser system that combines the characteristics of a fine spot laser source to process areas which require fine detail screening and a broad spot laser source for portions of the image where features comprise large solid areas.
An apparatus and methods for utilizing fine spot laser source 408 is shown in FIG. 4, as well as a broad (coarse) spot laser source 412 in a single imaging device 400 (partially shown) to achieve combined characteristics for spot and fine spot imaging, is described in the U.S. Publication No. 2008/0153038.
The two laser sources may be fixed on an imaging carriage 416 in a predefined distance 432 between each other. The imaging carriage 416 is moving substantially in parallel, along the longitudinal axis of the rotating drum 404 in direction 420, directed by direction screw 428. The imaging sources 408 and 412 configured to image on flexographic plate 104 which is mounted on rotating drum 404. The movement of imaging carriage 416 and imaging sources operation is controlled by controller 424.
Due to possible position deviations caused the movement of the imaging carriage 416 during the imaging process, the coarse imaging source 412 might image on areas intended to be imaged by the fine imaging source 408 and vice versa. The fine laser source is configured to image on areas on the plate planned to printed, therefore having the coarse laser source image on those areas might cause printing quality problems to appear during printing.
The current invention suggests an apparatus and method to overcome or at least to minimize those problems.